Until now, as a means for measuring a particular species of microbe included in a liquid such as drinking water, a cultural method has been used in general. This cultural method is a method of measuring the number of living microbes by applying a suspension liquid of a liquid sample to a culture medium, cultivating living microbes, and measuring the number of colonies made by cultivated microbes. This method also enables measuring and identifying a particular species of microbe by using a culture medium in which only a particular species of microbe can grow proliferously.
However, this cultural method requires a long inspection time to obtain a measurement result, since it takes one day to some days for microbes to make colonies. Besides, because an inspection process including suspension, dilution, application, colony measurement, etc. is manually performed, inspectors are required to have expertise and skill.
So, as a means for identifying a species of microbe rapidly, there is a method that takes advantage of difference in fluorescence spectra of microbes. This method takes advantage of the fact that different species of microbes have different types of substances in different amounts and are stained to different degrees by a fluorescent dye and, thus, different fluorescence spectra are obtained according to the species. Different fluorescence spectra are also obtained by varying the wavelength of an excitation light. By obtaining fluorescence spectra that are expressed using the wavelength of an excitation light as a parameter (hereinafter referred to as two-dimensional fluorescence spectra), it is possible to determine a species of microbe in more detail.
As a means for obtaining fluorescence spectra of microbes, a fluorescence spectrometer is generally used. The fluorescence spectrometer is an apparatus capable of irradiating a liquid contained in a special measurement cell with an excitation light with a particular wavelength and obtaining a spectrum of fluorescence light that is emitted from the liquid. To obtain fluorescence spectra of particles such as microbes, measurements can be taken by suspending these particles in a liquid such as ultrapure water.
An excitation light of the fluorescence spectrometer is a particular wavelength light drawn from light of a white light source through a grating (diffraction grating) and a slit. The wavelength of the excitation light can be varied by changing the position of the grating. Therefore, it is also easy to obtain two-dimensional fluorescence spectra that are taken by varying the wavelength of an excitation light as a parameter.
However, because a fluorescence spectrum that can be obtained by the fluorescence spectrometer is an average of fluorescence lights that are emitted from all the particles in a liquid, it is impossible to obtain a fluorescence spectrum of each individual particle. So, in order to identify a state and species of a single particle, methods of obtaining a spectrum of fluorescence or Raman scattered light that is emitted from a single particle have recently be reported.
A flow site meter based on Raman detection described in Patent Literature 1, which is mentioned below, obtains a spectrum of Raman scattered light by irradiating a particle which is an object of measurement and flows in a flow path with laser light and detecting, with a multi-channel detector, Raman scattered light that is emitted from the particle and dispersed through a dispersion element such as a grating.
A High-Throughput Single-Cell Fluorescence Spectroscopy described in Nonpatent Literature 1, which is mentioned below, obtains a fluorescence spectrum by irradiating a particle which is an object of measurement and flows in a flow path with 488 nm argon ion laser and detecting, with an ICCD (Intensified charge-coupling device), fluorescence light that is emitted from the particle and dispersed through a grating.